Electrical devices comprising conductive polymer compositions

Abstract

Electrical devices which comprise a PTC element composed of a PTC conductive polymer composition and a contiguous CW element composed of a conductive polymer composition which comprises an organic thermoplastic polymer and a conductive carbon black having a particle size (D) in millimicrons and surface area (S) in m 2 /g such that S/D is at least 10. D is preferably less than 27 millimicrons, especially less than 18 millimicrons. S/D is preferably at least 12, especially at least 18. Particularly useful devices are in the form of heaters.

I claim: 
     
       1. An electrical device comprising (a) A CW element composed of a CW composition which comprises (i) a continuous phase of a first crystalline organic thermoplastic polymer and (ii), dispersed in said first polymer, a first conductive carbon black having a particle size (D) which is less than 27 millimicrons and a surface area (S) in m 2  /g such that the ratio S/D is at least 12;   (b) a PTC element composed of a PTC composition which comprises (i) a continuous phase of a second crystalline organic thermoplastic polymer and (ii), dispersed in said second polymer, a second conductive carbon black, said PTC element being electrically and structurally directly bonded to the CW element; and   (c) at least two electrodes which are connectable to a source of electrical power and which are so placed in the device that, when they are connected to a source of electrical power, current flows between the electrodes through the device along a path which, at least at some temperatures, passes sequentially through said PTC element and said CW element.   
     
     
       2. A device according to claim 1 wherein the CW composition contains 6 to 40% by weight of the first carbon black and has been prepared by a process which comprises mixing the first carbon black with the first polymer while the first polymer is molten. 
     
     
       3. A device according to claim 1 wherein the first carbon black has a particle size below 18 millimicrons. 
     
     
       4. A device according to claim 3 wherein the first carbon black has a particle size of at most 15 millimicrons and a surface area of at least 300 m 2  /g. 
     
     
       5. A device according to claim 3 wherein the first carbon black has a surface area of at least 500 m 2  /g. 
     
     
       6. A device according to claim 1 wherein the ratio S/D is at least 18. 
     
     
       7. A device according to claim 1 wherein the ratio of the maximum resistivity of said CW composition in the temperature range from 25° C. to a temperature 40° C. below the melting point of said first organic polymer to the resistivity of said CW composition at 25° C. is less than 2. 
     
     
       8. A device according to claim 7 wherein the ratio of the maximum resistivity of said CW composition in the temperature range from 25° C. to the melting point of said first organic polymer to the resistivity of said CW composition at 25° C. is less than 5. 
     
     
       9. A device according to claim 1 wherein the CW and PTC compositions are cross-linked. 
     
     
       10. A device according to claim 1 wherein the resistivity of the CW composition at 25° C. is from 1,000 to 10,000 ohm.cm. 
     
     
       11. A device according to claim 1 wherein the resistivity of the CW composition at 25° C. is more than the resistivity of the PTC composition at 25° C. 
     
     
       12. A device according to claim 1 wherein the first polymer has a melting point T 1  and the second polymer has a melting point T 2  which is from (T 1  -25) to (T 1  +25)°C. 
     
     
       13. A device according to claim 1 wherein the first polymer is a copolymer consisting essentially of units derived from at least one olefin and at least 10% by weight of at least one olefinically unsaturated comonomer containing a polar group and the second polymer is a polyolefin. 
     
     
       14. A device according to claim 13 wherein the first polymer is a copolymer of ethylene and vinyl acetate or an acrylic ester and the second polymer is polyethylene or polypropylene. 
     
     
       15. A device according to claim 1 wherein the first and second polymers each contain at least 50% by weight of vinylidene fluoride units.